Clearly it can't be worse for your identity than losing all your limbs or becoming a quadriplegic?

Well, this is not clear, though it might be true.

I have frequently had the experience of not doing anything with my left leg; losing the ability to ever do anything with my left leg means I'm prevented from ever doing anything with it. This is horrible, of course, but it's the horror of being prevented from doing things I often choose not to do. Losing all my limbs is a more extreme version of the same thing.

Having different limbs might be more identity-distorting, by virtue of providing experiences that are completely unfamiliar.

Then again it might not.

For my own part, I'm not all that attached to preserving my current identity, so I'm not sure the question matters to me. If my choice is between an identity-altering pony body, and an identity-preserving quadriplegic body, I might well choose the former.